Heated intake air has been shown to provide a fuel economy benefit (e.g., 1.6%) from reduced pumping losses, and may also provide faster engine warm-up. In one approach, this may be accomplished via coolant heating of engine intake air. In such a case, intake air may be warmed via an exhaust gas recirculation (EGR) cooler.
The inventors of the present application have recognized a problem in such previous solutions. First, the maximum coolant temperature (e.g., 230° F.) may limit the amount of heat that can be provided to the engine inlet air. Second, the relatively slow warm-up of the coolant may limit the portion of the trip time that may be utilized to heat intake air.
Accordingly, in one example, some of the above issues may be addressed by intake air heating and exhaust cooling, wherein a double wall exhaust manifold may be configured as an exhaust-to-air heat exchanger. When the intake manifold pressure is less than ambient pressure, the engine can benefit from heated intake air. In such a case, fresh air may be drawn through an interstitial space of a double wall exhaust manifold to heat the air, and then the heated air may be directed to an intake manifold. As such, heated air is sourced to the intake manifold for intake stroke pumping benefit. In this way, by increasing the air heating with the hotter-than-coolant exhaust surfaces, the fuel economy benefit can be further enhanced. Moreover, ample exhaust heat is typically available in less than one minute after start, compared to three minutes or more for coolant heat.
Further, the inventors of the present application have recognized that the double wall exhaust manifold may additionally serve as an exhaust manifold cooler, by routing excess boost air through the interstitial air space, to cool the exhaust manifold during high load operation. As such, liquid cooling via an integrated exhaust manifold may be eliminated. Such cooling may be beneficial when the intake manifold pressure is greater than ambient pressure and the exhaust temperature is nearing a threshold associated with component durability. In this way, by cooling the exhaust manifold with air derived from excess boost, the fuel economy and emissions penalty of cooling via fuel enrichment can be reduced.
In this way, the double wall exhaust manifold as described herein establishes a synergy in functionality, in that intake air can be drawn in precisely when intake heating is desired, and the excess boost can push air precisely when exhaust cooling is desired.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.